In order to produce MRT images having the best possible signal-noise ratio (SNR), reception antennas have been placed as close as possible to the object to be measured (e.g., patient or subject). These are what are known as local coils, which may be in the form of reception coils but additionally may also be transmission coils.
For the user of the MRT, a local coil is a physical unit that he may position on, below or against the patient. The physical unit may have a plurality of logical subunits, the coil elements, which the user may individually select or deselect for the measurement, for example. A coil element in turn may combine a plurality of reception antennas that produce the individual MRT signals (the “raw data”), which are ultimately used in the MRT imaging as single and independent signals in order to reconstruct the image data therefrom. In this case, by way of example, a local coil has three individually addressable coil elements that each have six possibly individually addressable reception antennas, which provides that in this example a local coil would then have 18 single reception antennas.
A multiplicity of such reception antennas may allow for faster MRT measurements by parallel imaging methods, for example. Therefore the higher the antenna density of the reception antennas, that is to say the number of reception antennas for the local coil, the higher the image quality and/or the faster the raw data acquisition. The problem is that the large number of reception antennas produce a large number of independent signals that need to be transmitted via a large number of cables and digitized via a large number of A/D converters (analog-digital converters).
The problem with signal selection devices from the prior art is that the digital selection chips need to have at least as many signal inputs for logical data streams as A/D converter chips, even if only a subset thereof is ever selected in all data capture situations. This requires a correspondingly large number of chips for the digital selection chips.
In MRT systems to date, the number of logical signal inputs of the digital selection chips has always been chosen to be the same as the logical number of signal inputs of the A/D converter chips. If the number of signal inputs of the digital selection chips may not be increased arbitrarily, the number of A/D converter chips is limited to the number of signal inputs of the digital selection chips. Multiplexing provides that the physical number may differ from the logical number. Multiplexing in the analog domain is independent of that in the digital domain and dynamic for each actual cable.
In MRT systems, it may be necessary for the number of logical data streams from the A/D converter chips to be larger than the number of logical data streams that may be captured simultaneously with the current data capture process (e.g., readout).